NOVEL SYNTHESIS OF PHENOL DERIVATIVES BY PALLADIUM-CATALYZED CYCLOCARBONYLATION OF 2,4-PENTADIENYL ACETATES

Phenyl acetates were selectively obtained in good yields by cyclocarbonylation of 2,4-pentadienyl acetates in the presence of NEt3, Ac2O, and a catalytic amount of palladium complexes such as PdCl2(PPh3)2 at 120-140-degrees-C under 50 atm of CO. No five-membered ring products were observed. A platinum complex PtCl2(PPh3)2 was also effective as a catalyst. The reaction of 5-phenyl-2,4-pentadienyl bromide with M(CO)(PPh3)3 (M = Pd or Pt) under CO gave the corresponding 6-phenyl-3, 5-hexadienoyl complexes in a high yield, which in turn afforded 2-acetoxybiphenyl in 41-51% yield on heating to 160-degrees-C under 50 atm of CO in the presence of NEt3 and Ac2O. Similar 3,5-hexadienoyl complexes are proposed to be intermediates in the catalytic cyclocarbonylation of 2,4-pentadienyl acetates. On the other hand, PdCl2(PPh3)2-catalyzed carbonylation of o-(bromomethyl)(1-alkenyl)-benzenes in the presence of NEt3 and Ac2O gave 2-naphthyl acetates in moderate yields, while the reaction in the absence of Ac2O gave five-membered ring products such as 2-indanones or a tricyclic lactone by incorporation of one or two CO molecules, respectively.